Stainless steel containing bismuth



Patented Feb", 1942 Columbus, Ohio, assignors to Alloy Castin Institute,of New York, an unincorporated association of New York no Drawing.Application .Ianuary 17, 1940, Serial No. 314.244

6 Claims. (Cl. 75-126) This invention relates to stainless irons andsteels containing bismuth and more particularly to austenitic stainlesssteels containing a small but eflective quantity of bismuth for thepurpose of improving the machinability oi the stainless steels and theirresistance to corrosion.

We have found that the machiuability of stain- I less steels can bematerially increased by the inrelativelysmall but effective proportionof bismuth. I

It is a further important object of this invention to provide astainless steel of improved resistance to corrosion.

. Other important objects of this invention will bccomeapparent from thefollowing description and appended claims.

Our invention is applicable senerally'to any of the so-called "stainlesssteels," either the straight chromium or the chromium-nickel steels. Weprefer, however, to employ the austenitic stainless steels, containingboth chromium and nickel. Chromium may be as high as 30% and is usuallypresent in amounts of 12% or more. The stainless steels may be either inthe cast or wrought condition;

The amount of bismuth that may be added to a stainless steel to improveits macl'iinability lies,

in general, between the limits of 0.02 and 1.0%, preferably within thenarrower limits of from 0.1 to 0.6%. The bismuth can be added in anysuitable manner, but most advantageously may be incorporated into thestainless steel just prior to pouring, or while in the ladle, sincebismuth has a lower boiling point than the melting point of stainlesssteels and therefore tends to volatilize out of the molten metal,probably in the form of bismuth trioxide.

In order to incorporate as much as 0.6% of bismuth in the stainlesssteel, it may be necessary to add a much larger proportionate amount ofbismuth in the first instance, owing to loss by volatilization. Thepercentages of bismuth in the stainless steel, as given herein, arethose that maybe found by analysis of the stainless steel. Also. whileit is believed that bismuth is retained in the stainless steel largelyin its elemental form, the term bismuth" refers to the element itself,regardless of the form in which it may be present in the stainlesssteel.

. Heats of stainless steels with and without bismuth additions have beenmelted and subjected to machlnability tests. The tests were made bysawing and drilling the steels under rigorously controlled conditions inautomatic machines. The results are expressed as a "saw index" and adrill index." An index of 200, for example, indicates that the materialmachines twice as easily in that particular type of machining operation,as one whose index is 100. The remarkable and-heretofore unrecognizedeffect of small amounts of bismuth canbe seen' in the data for 5 sixseries of stainless and manganese steels of I further been substantiatedby machine shop difierent types shown in Table I. The efiect has latheobservations.

ticeable under corrosive conditions that gives rise to a pitting type ofattack. This has been demonstrated byqualitative tests of a large numberof these steels in an aqueous solution containing ferric chloride andsodium chlo- A ride. A 19% chromium, 9% nickel alloy is severely pittedin this solution in 2 hours.

TABLE I Composition I Saw Drill C M11 S1 S P Cr N1 B1 index index SERIES1 SERIES 11 0 11 0. 82 0. 0. 02 0. 01 25. 0 12. 0 0. 00 100 0.11 0.020.95 0.02 0.01 25.0 12.0 0.11 112 132 0. 11 0. s2 0. 95 0. 02 0. 01 25.0 12. 0 0. 13 133 147 0. l3 0. 82 1. 15 0. 02 0. 01 25. 8. 12. 2 0. 25142 0. 12 0. 88 1. 10 0. 02 o. 01 20. 0 12. 0 0. 37 205 SERIES IIISERIES IV SERIES v 0. 00 0. 50 0. 30 12. 0 0. 0 0. 00 100 0. 00 0.50 0.so 12. 0 0.0 0.10 SERIES v1 1.10 14.0 0.40 0.02 0.02 0.0 100 1.10 14. 00. 40 0. 02 0. 02 o. 20 1, 100 I 1 Maximum.

It will be noted that the tests of Series VI re- 50 TABLE 11 late to amanganese steel. Any manganese steel, that is, a steel containingbetween-about 11 and c Weight loss in 15% of manganese, exhibits amarkedly improved ompw g 9 i /gig machinability upon the additionthereto of a 501011011, 00:11:00- small percentage of bismuth. 55 andagitated.

c M 1 P 1: Ni Bi 7211 Tests of samples of snmlar steels with and n s r 102 without bismuth additions indicate increased corrosion resistancewhen small amounts of bismuth 0.11 0.82 0.95 0.02 0.01 25.0 12.0 0.00372.0 are present. The improvement is particularly no- The benefitsresulting from the addition of bismuth to stainless steels areaccompanied by a negligible change in the tensile properties of thesteel. This is exemplified in the tensile data of Table 111.

TABLE III Composition Alloy N0.

CMnSi s P 01 N131 Tensile prop rties (average) rm Yield m Reduc- AlloyNo. tio f $0.29}, Tensile tioni: -tionin limit d orma.) 21h. area Rm.p.11. P. .0. Percent Per cent 1 23,000 30,000 10,200 0202. 00.0 2 22,000Y 30,200 10,000 00.0 12.3 a 10,500 00,400 09,000 01.0 14.0

It will, of course, be understood that various details of compositionmay be varied through a wide range without departing from theprinciples-of this invention and it is, therefore, not the purpose tolimit the patent granted hereon otherwise than necessitated by the scopeof the appended claims.

1 We claim as our invention:

1. A ferrous ,alloy containing from 0.02 to 1.0-

per cent bismuth, an amount of alloying material suflicient-to renderthe structure of the alloy austenitic and selected from the groupconsisting of nickel, nickel and chromium, and manganese, and theremainder substantially all iron,

said bismuth content effectively improving machinability oi the alloy.2. An alloy having improved maehinabllity con taining 12 to 30 per centchromium. 0.02 to 1.0 per cent bismuth and the remainder substantiallyall iron.

3; An alloy having improved machinability containing 12 to 30 per centchromium, 8 to 12.6 per cent nickel, 0.02 to 1.0 per cent bismuth, andthe remainder substantially all iron.

4. An austenitic steel alloy havingimproved um, and manganese, andcontaining from 0.02

to. 1.0fper cent bismuth, said alloy having improved machinability dueto the said presence of bismuth therein. I I

ROBERT S. PEOPLES.

HENRY A. HOLDEN PRAY.

